The present invention refers to a method of varying the application amount of coating agent at bladecoating of a moving material web, preferably a paper web, by corresponding control of the engagement pressure of the coating blade against the web, which is supported by a support, preferably a rotating support roller, and which exhibits a coating agent surplus, said blade being along one of the longitudinal edges thereof clamped into a blade holder, the opposite longitudinal edge of said blade being initially brought into engagement with the web by means of a control pressure acting against the main surface of the blade, whereby the blade has a curved shape and forms with the tip portion thereof a certain predetermined angle with the web, so that a certain area of engagement is defined or formed. The invention also refers to an apparatus for carrying the method into effect.
In bladecoating devices the application amount of the coating agent is regulated by the engagement pressure of the blade against the paper web, and the pressure per surface unit for the engagement area between the blade and the web, i.e. the work area or engagement area, substantially determines the application amount of the coating agent. The blade pressure is often regulated thereby that the distance between the bladeholder and the support is changed. Normally the engagement surface or work surface of the blade normally extends namely in parallel with the web surface and the roller surface respectively. However, when the bladepressure is varied in the way described above, this parallelity is lost, because a change in the blade deflection occurs and thereby a change of the engagement area between the blade and the web also occurs. Thus, when it is decided to change the application amount by means of this known technique, the bladepressure against the roller will increase if the distance between the roller and the bladeholder is decreased, but at the same time the angle between web and blade top will decrease. This means initially that the surface of the blade will form an oblique angle relative to the paper web whereby only a portion of the surface will be in engagement with the web. The engagement pressure per unit of surface area, which determines the application amount will then increase substantially, causing a decreased application amount. By and by as the blade is worn in, the engagement portion of the work surface with the web will increase until the entire work surface area is in engagement with the web. During this period the application amount will increase. When the blade again has been worn in, the entire engagement surface will engage the web.
As appears from the above example, this type of regulation has two drawbacks:
1. For each change of the distance between the blade holder and the roller, the blade will have a time period of wearing in during which the application amount is not constant.
2. The changes in the application amount will have different action depending on various factors such as the springing properties of the blade material, the protruding length of the blade, the thickness of the blade and the initial top angle of the blade.
Further to this there are various methods of changing the bladepressure. The majority of these methods is burdened with the drawbacks described above.
Moreover, it has previously been suggested to solve the present problem by turning the bladeholder around the intersection point between an imaginary line which forms an elongation of the straight part of the clamped blade, and an elongation of that part of the blade which is in engagement with the paper web. This known solution is mechanically complicated and thereby utterly sensitive as to function, although, with certain preconditions, it gives the opportunity to change the bladepressure without change of the bladeangle. However, with this known technique it is only possible to achieve a uniform pressure regulation along the entire length of the blade. On the other hand this technique excludes the possibility of so called profile regulation--i.e. a change of the application amount across the web. The demand for such a profile regulation is rapidly increasing, i.e. dependent on increased web velocities and increased application amount and higher demands for quality.
The object with the present invention is to provide a method and a device which in a satisfying way makes possible a simple and functional regulation, for example continuous fine adjustment of the application amount during the application without changing of the engagement area of the blade against the web, and also a simultaneous possibility to desired variations of the application amount across the web--i.e. possibilities to a so called profile regulation.
According to the invention this is achieved with the method described in the preamble herein substantially thereby that the engagement pressure of the blade against the web, with a substantially maintained initial engagement area against the web, is controlled by external force application along the concave bladeside of the blade between the clamping position and the engagement area.
According to a suitable embodiment of the invention, said force application on the blade occurs along a path which substantially coincides with the arcuate shape of the blade. Said blade is then suitably substantially shaped as an elliptical or circular arc.
According to one embodiment of the invention said force application suitably occurs simultaneously along the entire length of the blade, and this is achieved by means of a pressure means of a support means which is displacable along the concave side of the blade. The pressure means or support means can then be displaced manually or be for example computer controlled depending on the application amount which is sensed each time.
According to another embodiment of the invention varying force application is performed along the length of the blade, and this is suitably achieved by means of a number of partial pressure means or partial support means arranged in a line along the entire length of the blade, said partial pressure means or partial support means being independently controllable.
Suitably said varying force application on the blade can be provided via a rod which is flexible at least in one plane and which is located between the pressure means and the blade side facing said means, said rod being arranged along the entire length of the blade and arranged in engagement with the blade.
The invention also comprises a device for carrying into effect the preamblewise described method, said device being substantially characterized by pressure means or support means arranged to cooperate with a predeterminable pressure with the blade along the concave side thereof.
According to a suitable embodiment, the pressure means or support means comprises a support roller which is arranged turnable, and a support which is arranged to be adjustable along a path substantially coinciding with the arcshape of the blade, said support roller extending along the entire length of the blade.
Suitably the support roller is fastened on a rotatable shaft which is arranged at the blade holder, said support being arranged adjustable along a substantially circular path.
According to a further embodiment of the invention the device comprises a number of parallel pressure means or partial support means arranged in line after each other along the length of the blade and each arranged to be brought to point engagement with the blade.
Suitably a rod which is flexible at least in a plane and which extends along the entire length of the blade, is arranged in mechanical contact between the concave side of the blade and the pressure means.